In applications for pulsed laser beams such as isotope separation and in particular uranium enrichment, examples of which are shown in U.S. Pat. Nos. 3,772,519; 3,939,354; 3,924,937; and 3,944,947, pulsed radiation is employed to produce isotopically selective photoexcitation and/or ionization of a vapor environment of the particles of plural isotope types, one or more types of which are to be separated. In providing the laser radiation pulses for this application, it is common to generate a beam of a precisely defined spectral purity to insure selective excitation and to provide amplification in one or more stages of amplifiers to a useful energy level. Laser radiation pulses generated in such a manner typically have a conventional bell-shaped pulse characteristic. The slow rise and fall shape is typically too long in duration for the period of time during which useful photoexcitation may take place, typically approximately one microsecond. The excessively slow rise time necessitates wasting or undesirable preapplication of the leading edge of the radiation pulse until a sufficient intensity for efficient photoexcitation is reached. This can result in an undesirable loss of radiation energy and efficiency.